There are numerous methods for machining toothed wheels. In the chip-producing production of spiral bevel gears, a distinction is made between the single indexing process and the continuous process, which is also sometimes referred to as the continuous indexing process.
In the continuous process (also referred to as face hobbing), for example, a cutter head tool comprises inner cutters and outer cutters, which are arranged group-wise, and is applied to a workpiece for cutting the convex and concave flanks of the workpiece. That is, the workpiece is completely cut in a single clamping in the non-stop process. The continuous process is based on very complex coupled sequences of movements in which the tool and the workpiece to be machined perform a continuous indexing movement. The indexing movement results from the coordinated driving of a plurality of axle drives of a corresponding machine. In the continuous indexing method, the rotation of the cutter head and the work-piece to be machined are coupled such that each time only one cutter group moves through a tooth gap and the next cutter group moves through the next space. The indexing is thus carried out continuously and all gaps are generated virtually simultaneously. By these coupled movements, an extended epicycloid results as a longitudinal flank line on the crown gear of the bevel gear to be produced. In the continuous process, the cutters of a cutter group are arranged one behind another with a phase angle, wherein the cutting edges of the outer and inner cutters intersect in a common projected plane.
In the indexing process (also called single indexing process or face milling), one tooth gap is machined, then a relative displacement movement is carried out for backing out the tool from a tooth gap and thus allowing a so-called indexing movement (indexing rotation), in which the workpiece rotates relative to the tool before the next tooth gap is machined. Thus, a toothed wheel is fabricated step by step. In the single indexing process, a first cutter head comprising inner cutters and outer cutters can be applied for cutting inner flanks (convex tooth flanks) on the workpiece and for preliminarily machining outer flanks. The outer cutters do not produce the final geometry of the outer flanks. Then, the first cutter head can be replaced by a second cutter head provided with outer cutters for cutting the final outer flanks (concave tooth flanks) on the workpiece. This procedure is also called single-side cutting. The cutting edges of the tool are arranged circularly (e.g. for a face cutter head) and the flank lines generated on the workpiece thus have the shape of a circular arc.
In the single indexing process described, the replacement of a cutter head takes place, which leads to a prolongation of the total machining time duration on one hand and that can involve inaccuracies on the other hand, as each clamping change or new clamping can lead to small deviations from the ideal position. The indexing process further has the disadvantage that it involves so-called indexing errors. It is an advantage of the single-side single indexing process involving two separate cutter heads that both tooth flanks can be optimized virtually independent from each other.
The so-called completing process is a special single indexing process, which is preferably employed in mass production. In the completing process, the crown gear and the pinion gear are machined and completely finalized in a two-flank-cut. With respect to other single-indexing processes, the completing process is characterized by a higher productivity (double chipping capacity); however a change of the flank shape is more difficult because changes in the kinematics of the machine will always have an influence on both flanks, just as with all processes comprising a two-flank-cut. It is thus a disadvantage of the completing process that a change of a flank by means of the kinematics of the machine always involves a change of the other flank. Changes are therefore possible only if they are “compatible with two-flank-cutting.”
Accordingly, a distinction is essentially made also between machines working according to the indexing process and machines working continuously.
The so-called multicut or OERLIKON multicut gear cutting process, which is described, for example, in the document “OERLIKON Spiralkegelräder, Berechnungen, Herstellung and Optimierung” (in English: “OERLIKON spiral bevel gears, calculations, manufacturing and optimization”), Schriftensammlung (English: collection of documents) 1988/89, Werkzeugmaschinenfabrik Oerlikon-Mihrle AG, OERLIKON Ver-zahnungsmaschinen (English: OERLIKON gear cutting machines) on pages 73 to 77, is denominated as the closest state of the art.